The broad, long-term objectives of this program are: The development of diagnostic tests for the detection of minimal residual disease. The incorporation of such tests into protocols that improve patient survival after bone marrow transplantation. The specific aims of this project are: 1) Determine whether the polymerase chain reaction (PCR) can detect minimal residual disease in patients transplanted for CML. 2) Identify which cells give rise to PCR positivity in remission CML patients. 3) Determine the utility of PCR to detect minimal residual disease in patients with ALL. Patients will be evaluated by PCR prior to BMT and at defined intervals after BMT Existing PCR methods for detection of BCR-abl mRNA rearrangements will be applied to patients with Philadelphia (Ph) positive CML and ALL. Patient specific immunoglobulin markers will be used for PCR studies of the Ph- negative ALL patients. Many posttransplant patients will relapse. Conventional technologies can not distinguish who is truly cured and who will relapse. Studies outlined in this project should allow for the identification of these two patient populations and lay the foundation for early intervention that may ultimately lead to increased overall transplant survival.